THE LAW OF MINIMUM. 211 



been the minimum, after manuring with superphos- 

 phate, a suitable supply of potash or ammonia, or both, 

 would have increased the produce. In this same series 

 of experiments, 6 cwt. of guano, corresponding to 2 cwt. 

 of superphosphate, gave a crop of 630 cwt. of turnips, 

 or 130 cwt. more than the superphosphate ; but it is 

 left in doubt whether this increase was attributable to 

 the potash or the ammonia in the guano. 



To return to our Saxon experiments. If we look at 

 the different quantities of dung applied severally on 

 the five fields, we are naturally led to inquire the 

 reason of this diversity. 



The most feasible answer, perhaps, is, that the far- 

 mer gives as much manure as he has at his disposal, or 

 that he regulates the quantity according to certain 

 facts. If he has found by experience that a certain 

 quantity of farm-yard manure will restore his land to 

 its original fertility, and that more copious manuring 

 will fail to give larger crops, in proportion to the addi- 

 tional supply, or to the cost incurred in collecting the 

 manure, he will stop at the smaller quantity. 



Hence it cannot be regarded as a mere accident that 

 the farmer at Cunnersdorf contented himself with 180 

 cwt. of farm-yard manure, while the farmer at Ober- 

 bobritzsch laid 314 cwt. upon his field. 



But if the quantity of manure to be applied is not 

 dependent upon chance or caprice, but is regulated by 

 the object in view, it is manifest that the proceedings 

 of the farmer are governed by a law of nature unknown 

 to him, except by its effects. 



It is in the composition and condition of the soil 

 that we must seek the law which regulates the quantity 

 of farm-yard manure required, at the outset of a fresh 

 rotation, to restore a field to its former fertility ; and it 

 is not difficult to see that this quantity must always be 

 proportionate to the effective dung-constituents already 

 present in the soil ; a field largely abounding in them 

 takes less manure than a poor field to give the same 

 increased produce. 



Now, as farm-yard manure owes its most active 



